Current investigations concern the class I and class II genes in the major histocompatibility complex of the rabbit. The class I gene family contains approximately 10 class I genes falling into four subfamilies. Structural data are available for four rabbit cDNA clones, and the genomic clones corresponding to three of these have been completely or partially characterized. Using probes derived from these clones, it was found that the clone pR27 corresponds to a single copy rabbit class I gene that is expressed in the T cell line RL-5 and also in normal rabbit thymus, but not in the other tissues studied. The pR27 transcript is larger than normal class I transcripts apparently because certain introns have not been spliced out. Studies are under way using S1 nuclease maping to determine the precise splicing patterns in the transformed cell line (RL-5) as well as in normal thymus. Studies of rabbit class II genes have included complete sequence analysis of a cDNA and a genomic clone corresponding to the rabbit DQ alpha homologue. This gene is highly homologous to the HLA-DQ of man and is quite similar to the H-2 I-A alpha gene of the mouse. The RLA-DP2 alpha gene has been partially sequenced and data indicate that it may be a homologue of the human gene designated HLS-DZ.